The invention relates to a contact element for an electrically conductive connection between an anode and an interconnector of a high-temperature fuel cell. With high-temperature fuel cells, it is customary to combine individual fuel cells to stacks and to connect them to one another in an electrically conductive manner in so doing. For this purpose, interconnectors, which are also called bipolar plates, are arranged between the electrodes of individual fuel cells. The electric current can thus flow from one electrode of a fuel cell to the oppositely poled electrode of the proximate fuel cell.
Since it is, however, necessary that fuel or oxidizing agent can move to the electrodes, a permeability or porosity is required for this purpose, which influences the electrical conductivity.
Differently configured current collectors have thus previously been used between the electrodes and the respectively arranged interconnectors. Examples for this are networks or current collectors which are formed with fibers (DE 102 32 093 A1).
A further problem which generally has to be taken into account with high-temperature fuel cells is the thermal expansion, due to the high operating temperature of the fuel cells. For this reason, as a rule the materials for all essential individual elements of the fuel cell, that is the material for the electrodes and the interconnectors, and in part also the materials of the current collectors, are selected such that the coefficient of thermal expansion of the solid electrolyte is taken into account. Only small deviations should be permitted in this respect.
The current collectors which provide the electrically conductive connection between an anode and an interconnector associated therewith are made from nickel as a rule. However, nickel has a much greater coefficient of thermal expansion. On the cooling down of a high-temperature fuel cell from the operating temperature to the room temperature, a length deficit is formed in the anode contacting. This has the result that a ceramic contacting of the cathode is subject to tensile strains and the contact can tear off there. Electrical losses thereby occur and the achievable power is reduced.
A reduction in the thickness of such an electrically conductive connection formed with nickel in this manner between the anode and the interconnector is also not expedient since mechanical strains and production tolerances must also be compensated by the ductility of the nickel.
On a formation of the contact between the anode and the interconnector using nets or fiber structures, a locally differentiated deformation also occurs during the operation of fuel cells which has the result that spots having increased electrical conductivity occur which are called “hot spots”. The larger part of the electric current flows there.
If it occurs in this process that the contact is disturbed at these positions, it necessarily results that the electrical conductivity is reduced and the electrical resistance is increased. This has a particularly disadvantageous effect on the restarting of a previously deenergized fuel cell since no such contact is present at a current collector formed in this manner at least during this time via which a large electric current can flow locally restricted in this manner between the anode and the interconnector.